(1) Field of the Invention
The present invention relates to a flowability improving agent which also gives a prolonged workability time of an aqueous cement containing composition, a process for the production of such an agent and a use thereof.
(2) Description of the Related Art
Flowability improving agents have been known for a long time as additives to compositions containing cement, lime and/or gypsum. Grouts, mortars, concrete, putty and self levelling floor screeds constitute examples of cement based mixtures. Usually these mixtures are composed in such a way that they contain a surplus of water as compared to the minimum amount necessary for the curing or binding reaction of the cement.
If one tried to use exactly the amount of water necessary for the curing, the mixture would get an unsatisfactory consistency and workability due to the strong attractive forces between the cement particles.
On the other hand, a too high water content in the mixture mentioned results in a drastic decrease of the strength of the cured mixture. However, to a certain extent this can be compensated by an increase of the cement content at the composition of the mixture. A too high content of cement per unit of volume has, however, the negative effect that the heat release at the curing reaction may result in crack formations and other less desirable effects. Also from an economical viewpoint it is undesirable to use a higher content of cement than necessary.
Nowadays, cement based mixtures are often pumped under pressure through a pipeline different building work sites. Then it is important that the mixture has a pumpable consistency but not too high a water content.
It is known before to use certain concrete additives to be able to decrease the water content of the concrete mixture and at the same time get a good flowability. The most well known of these additives are sulphonated naphthalene-formaldehyde resins, sulphonated melamine-formaldehyde resins, sulphonated melamine-urea-formaldehyde resins and sodium and calcium salts of ligninsulphonic acid.
Thus, these known additives give a certain effect in the respects mentioned above.
The conception of workability time is used in the present technical field to define the period of time after mixing a flowability improving agent of the above mentioned type with a concrete mixture, during which the mixture has a better flowability than the corresponding concrete mixture without any admixture of a flowability improving agent.
Different methods can be used to determine the workability time. According to the present patent application a standardized Swedish method, SS 137121 is used for consistency measuring. Therein a metal cone is filled with a concrete mixture whereupon the cone is lifted up and taken away leaving the unsupported cone of concrete. Then the concrete cone sinks down.
The difference between the height of the concrete cone and the metal cone, the so-called slump is used as a measure of the consistency of the concrete. Then the slump is measured on a concrete mixture before the admixture of a flowability improving agent and after the admixture thereof. Samples are taken out at even intervals until the slump of the concrete mixture containing the flowability improving agent has reached the same value as the concrete mixture had before the addition of the agent. The time period measured constitutes the workability time.
Today, concrete producers and building contractors often have very big problems with a too short workability time of the concrete. Thus, the effect of the flowability improving agent in the concrete decreases too quickly. Sometimes, for instance the transport of the concrete from the concrete factories to the building sites takes too long preventing the concrete from filling the molds etc before it loses its flowability. Moreover, on the building site pouring stops can occur because of different reasons. When the pouring then starts again the concrete has lost a great deal of its workability. This causes big problems especially if pumping of flow concrete is involved. A plugged pump conduit can result in very big economic consequences.
The problems with a too short workability time are particularly big at high concrete temperatures during summer time and with pouring with warm concrete during winter time.
Of the above flowability improving agents, above all sulphonated melamine-formaldehyde resins and sulphonated naphthalene-formaldehyde resins are used for flow concrete. Then the problems disclosed above occur.
Very big research efforts have been spent in many places in the world to solve the problem with too short a workability time of concrete. In certain cases it has partially been possible to extend the workability time. However, at the same time other problems have appeared, such as a decreased short time strength, an increased air content, a risk of false set or an extremely prolonged time of initial setting of the concrete. These problems are so serious per se that it cannot be said that the problem with a short workability time has been solved in a satisfactory way before.